1. Field of the Invention
The present invention relates to a stop apparatus used in a lens apparatus for image taking and more particularly to a stop apparatus in which the aperture diameter is varied by a rotational operation. The present invention also relates to a lens apparatus and an image pickup apparatus having such a stop apparatus.
2. Description of the Related Art
There has been known heretofore a stop apparatus having a plurality of stop blades each having a pivot that are arranged in the same circumference at regular angular intervals so that a stop aperture defined by overlapping edges of the stop blades can be varied with swinging of the stop blades about the respective pivot shafts. In a typical stop apparatus of this type, each blade is provided with a pivot pin and a driving pin, and the position of the driving pin relative to the pivot pin, which is supported at a fixed position, is guided by a cam groove formed in a disk-like cam plate.
For example, Japanese Patent Application Laid-Open No. 2004-294678 discloses a stop apparatus in which pivot pins of six stop blades are fitted to engagement holes provided on a blade supporter, and the stop aperture size is varied by rotating a cam plate having six cam grooves engaging with the driving pins of the respective blades. When this stop apparatus is built in a lens apparatus, the cam plate is directly linked with a rotational operation member to allow manual adjustment of light quantity.
The shape of the stop aperture affects the quality of image blur characteristics (so-called “bokeh”) in background regions around the main subject in taken images. Specifically, the more circular the shape of the stop aperture is, the better the blur characteristics in images can be. A typically used method of making the stop aperture shape closer to a circle is to increase the number of stop blades. Japanese Patent Application Laid-Open No. 2012-123299 discloses a stop apparatus having an aperture shape made closer to a circle by the use of as many as fourteen stop blades.
For the above-described reason, stop apparatuses are desired to have a stop aperture that is close to a circle in shape and to have a large rotational operation angle in manual operation in order to allow fine adjustment of the light quantity. Both the aspects mentioned above are required particularly in the case of lens apparatuses that need to generate aesthetically fine images and to allow fine light quantity adjustment, as is the case with cinema lenses.
However, in the prior arts disclosed in the above-mentioned Japanese Patent Application Laid-Open Nos. 2004-294678 and 2012-123299, it is difficult to achieve a large rotational operation angle while using a large number of stop blades, because it is necessary to prevent the cam grooves for guiding the driving pins of the stop blade from interfering with each other. Consequently, the conventional stop apparatuses that use a large number of stop blades in order to make the shape of the stop aperture close to a circle cannot allow fine adjustment of the light quantity using an operation member. This deteriorates the usability of the lens apparatuses in image taking.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. H03-182709 there has also been known a structure that enlarges the rotation angle of a manipulated member relative to the rotation angle of a rotary member in a lens apparatus by means of a differential mechanism using rollers. However, the use of this structure in a stop apparatus leads to a problem of increased overall size of the stop apparatus including the differential mechanism.